Abstract : The aims of this research were to study each of the various molecular mechanisms whereby toxic metal cations and oxyanions are immobilized by bacteria that live in the soil. The research effort focussed on two bacteria: Xanthomonas maltophilia strain OR-02, an organism that chemically transformed individual metals to a less mobile state; and Pseudomonas mendocina strain AS302, a bacterium that formed tight complexes with a wide variety of toxic metal ions. The NADPH-dependent reduction of Hg(II) to elemental mercury by OR-02 was catalyzed by an inducible mercuric reductase. The reduction of selenite and tellurite to their insoluble elemental forms was mediated by an intracellular glutathione reductase that utilized the spontaneously-formed bis(glutathio)Se or bis(glutathio)Te, respectively, as pseudosubstrates. The 3-electron reduction of hexavalent chromium was catalyzed by a membrane-bound chromate reductase. A total of 14 different heavy metal ions were shown to bind tightly to strain AS302 with biosorption capacities that ranged from 320 (Pb) to 680 (Ag) mu-mol metal/g dry cell weight. This project could provide useful information toward the eventual exploitation of these organisms for the removal of toxic metal wastes from selected, heavily polluted sites.